Implantable medical devices (IMD) are commonly used to treat and monitor patients. Examples of an IMD include implantable cardiac pacemakers, implantable defibrillators, implantable pacemaker/cardioverter/defibrillators, implantable muscular stimulus devices, implantable brain stimulators, other implantable organ stimulation devices, implantable drug delivery devices, implantable monitors, and the like. These disparate medical devices often apply one or more medical treatments, and generate a wealth of physiological data relating to the condition of the patient.
This physiological data is often collected by use of telemetry, which generally refers to communication of data, instructions, and the like between an IMD and an external device, such as a medical device programmer or an external monitor. A communication “session” is established to communicate the physiological data to the external device, and to communicate any controls or commands from the external device to the IMD. In this manner, the external device may obtain diagnostic data, event marker data, activity data and other data collected or identified by the IMD. The physiological data may be used to program the IMD for delivery of new or modified therapies. As a result, telemetry between an IMD and the external device can be used to improve or enhance therapy.
To establish the communication session, the external device often examines a defined frequency spectrum to select one of a number of defined communication channels. During this process, the medical device programmer examines the channels individually in an attempt to select an open channel. If the external device detects activity on the channel, e.g., communication between another device and an IMD, the external device examines a different channel.
Once an external device and an IMD have established a communication session, the external device and the IMD communicate in a bi-directional fashion using downlink and uplink communication messages. The external device and the IMD alternate between downlink and uplink communication messages, respectively. In particular, the external device transmits downlink communication messages and receives uplink communication messages. The IMD receives downlink communication messages and transmits uplink communication messages. Each downlink and uplink message pair may form a single frame within the communication session.
During the communication session, the data exchange between the devices may vary greatly depending on a number of factors, including the amount of physiological data and programming commands to be transferred. For example, the external device and IMD may exchange uplink and downlink messages at a reduced rate when higher-rates are unnecessary, thereby reducing power consumption by the IMD. In other words, when there is less information to transmit from the IMD to the external device, the IMD transmits fewer uplink messages. This reduced transmission allows the implant to save energy associated with the transmit operations. Similarly, the external device transmits fewer downlink messages when there is less information to downlink to the IMD. This allows the IMD to save energy associated with the receive operation.
These reduced transmissions may cause significant communication gaps in the communication session, i.e., periods during which there is no communication between the devices. For example, a communication gap may occur when the IMD is collecting physiological data in real-time per the request of the external device. During these gaps, another external device may erroneously determine that the communication channel is unused, and may attempt to initiate a communication session. This usage may interfere with the previously established communication session, and may result in data loss, consumption of additional power by the IMD in attempting to respond to erroneous communications, and possibly premature termination of the original communication sessions.